When there is already a long-running thread pool for a work manager and you try to create another one:
/subsystem=jca/workmanager=default/long-running-threads=custom:add(max-threads=30, queue-length=30)
you only get an opaque error message:
"failure-description" => "WFLYCTL0086: Failed to persist configuration change: WFLYCTL0084: Failed to marshal configuration", with a, also useless, java.lang.IllegalArgumentException in the server log.
It should be more obvious that the error is that you cannot create two long-running thread pools